The present invention relates, in general, to the control of three-phase induction motors and, in particular, to a new and useful method and apparatus for generating digital pulse width modulated waveforms which can be used for the constant torque variable speed control of an induction motor for closed loop position control.
Previous methods utilize analog circuitry consisting of sinusoidal and triangular wave form generators to produce the pulse width modulated wave forms necessary to control a three-phase induction motor for constant torque variable speed control. The analog technique is usually complex, requiring numerous factory and field adjustments. The analog system is difficult to reconfigure to different applications after final design.
Limited use of digital techniques (including microprocessors) have been attempted for open loop induction motor speed control.
In U.S. Pat. No. 4,099,109 to Abbondanti, a digital apparatus for synthesizing pulse width modulated wave forms is disclosed. According to that reference, however, the wave forms are selected by hardware. The versatility of Abbondanti for adapting the induction motor control to various applications is thus limited. In addition, the provision of the required logic for the induction motor control using hardware limits the flexibility of such control, and the adaptability thereof to various different conditions and requirements.